• Finnish water environments are fragile.
• Eutrophication is caused by nutrient emissions from human agriculture, habitation, and industry.
• Eutrophication threatens water ecosystems by changing their delicate balance.
• Dams are a threat to river ecosystems.
• Acid rain caused by industrial toxins can cause the acidification of water ecosystems.
• Invasive species can threaten the survival of native species.
• Key words: eutrophication, lack of oxygen, cyanobacteria, acidification, hydraulic engineering, environmental toxins, invasive species.